The subject invention deals with submerged marine propulsion devices, such as propeller drives and submerged discharging jet drives. In particular, it relates to an "anti-Coanda" effect drive with a very low underwater noise signature. The subject invention, besides producing a high net thrust, also has the potential of lowering vessel required thrust to make a specific speed, both through "clean" water and through debris laddened waters, such as in ice laddened seas. It also provides a means wherein the propulsive force necessary to make speed may be quietly transfered into the water, thereby offering the vessel a greater stealth capability than may be available through comparable propulsive means. In military applications, this could be a mine sweeper operating about accoustically sensitive mines . . . a submarine countermeasures vessel operating with low propulsion system noise while attacking a target . . . a submarine operating in a "silent" mode . . . and types of torpedoes. It may also be advantageous to fishing vessels and research vessels, both for the gathering and studying of aquatic life.
Inboard auxiliary or main drive engines which use directed water for propulsive power, such as propellers and marine jets, develop thrust by the transfer of momentum, e.g., the ejection of water away from the boat system. With propeller drives and submarine discharging jets located submerged near the hull, the ejected water drags other water with it and influences the water flow about the hull. This "dragging" of water into a changed path about the hull takes applied thrust away from that available for driving the hull forward or driving objects away. A negative pressure zone of influence is created against the hull, and this in measure cancels a portion of the thrust capability of the propulsor.
The propensity for a moving fluid to follow a curved surface it is flowing against is known as the "Coanda Effect". Thrust lost through a propeller or water pumping device as compared with its test tank "model" test is composed of changes (powered vs. unpowered vessel) in (a) flow fields of the water pumping device due to installation in the vessel, (b) modification of the vessels frictional and eddy drag characteristics due to the water pumping devices changes in the vessels boundry layer flow path at speed, (c) modification of the vessels wave making, and (d) Coanda effect losses. The total cf these losses is called the "thrust deduction factor", or TDF. Simply stated, the thrust fraction (t) lost is the difference between the thrust required to free tow an unpowered vessel at speed as apposed to the shaft thrust required to push the vessel with the propulsor installed. The thrust fraction lost can range from a loss of a few percent to over 50 percent depending on the propulsor and the installation. TDF can be measured by subtracting the VESSEL ENGINED MEASURED THRUST (VEMT) from the IDEAL FREE UNPOWERED VESSEL TOW THRUST (IFUT) or vessel system model tow thrust, and dividing this by the IFUT. This fraction, representing thrust CONSERVED, must be subtracted from 1 and multiplied by 100 to yield percent loss, e.g.: EQU T percent loss={1-[(IFUT-VEMT)/IFST]}.times.100
The remaining thrust, or actual propulsive thrust, acts to drive the hull into equalibrium with hull resistance as the vessel accelerates to speed. The above assumes a high order of correlation between "model tests" and historical sea trials data. Ideally, IFUT is from a full scale model test.
The total thrust required to drive a vessel at speed, e.g., Thrust Horsepower (THP) is equal to the shaft thrust required to overcome the unpowered Ships resistance at speed (Sr). The NET horsepower driving the vessel is the EFFECTIVE horsepower, or EHP. More accurately defined, EQU THP={T.times.[Vk(1-w)]}/325.6 EQU EHP=[T.times.Vk.times.(1-t)]/325.6 EQU Hp. lost=THP-EHP=(THP).times.t EQU t=[THP-EHP]/THP
Where:
T=Propulsor thrust in lbs. PA1 Vk=Speed of ship through water in knots PA1 Va=Propeller speed of advance in knots PA1 t=Thrust Fraction PA1 (1-t)=Thrust Deduction Factor PA1 w=wake fraction
where EQU (Taylor) w=(Vk-Va)/Vk=1-(Va/Vk)
The resistance due to the ships underwater profile is composed of Frictional Resistance, Eddy Current Resistance, and Wave Making resistance. The subject invention operates to reduce thrust deduction factor (TDF). The subject invention operates, to a smaller degree, to reduce the vessels "free towed" driving thrust requirements by reducing the vessels stern wave making and stern hull resistance (due to local frictional and eddy current) properties. The subject drive not only reduces the influence of propulsion system water flow effects on the hull form, but also can change the "effective" hull shape under the influence of a fluid flow about a curved body.
The invention herein described is related primarily to submerged discharging water pumping systems used for powering marine vessels, such as a submerged discharging marine jet pumps, nozzled propeller drives or tube (tunnel) mounted propeller drives. The subject invention provides a means wherein heat from the motor or engine can be used to provide gases for surrounding the ejected submerged discharging jet stream for providing increased net propulsive thrust. Also, the motor or engine waste or exhausted gases can be vented around the outside of the ejected jet stream. Alternately, ambient air may be supplied to produce the gaseous boundry layer surrounding the jet stream and against the submerged surfaces of the vessel hull. Alternately, the gas supply to the subject invention may be supplied by a separate chemical gas generator means or compressed gas supply. The gases provided surrounds the submerged water jet or water stream and flows up against the vessel hull and/or flows with the jet or water stream away from the hull. This develops a barrier layer which expands and provides a low average kinematic viscocity shearing layer, thus working to reduce the gross propulsion system effort in thrusting the vessel forward (as to be hereinafter further explained), and in accoustically isolating the thrusting system (isolating propulsion system and jet discharge noise). Further, this gaseous boundry layer or shearing layer, besides increasing net propulsive thrust and through the modification of the vessels resistance properties, increases boat speed, it also significantly reduces propulsive force noise transmission into the water. This would make the drive useful in applications requiring great stealth, such as in types of submarine warfare, in vessels engaged in mine sweeping, and in types of fishing operations.
To reduce parasitic hull drag and deterioration of jet efficiency by marine life growing inside the jet pump, and for closing of the jet openings to lower hull resistance when the jet system is not in use (when the jet is a power augmentation or auxiliary power source), streamlining and sealing hull closures are incorporated. The jet system may be used in an auxiliary or thrust augmentation source on vessels which sail, on military and maritime vessels which have as their main power system a fixed blade or controllable pitch propeller system, and as a prime mover in its own right. The aforementioned system may be used as a propulsion system, as well as a bow thrusting system.